The present invention relates to an analog-digital converter, more particularly, to an on-die thermal sensor including the analog-digital converter.
An analog-digital converter is an apparatus for converting an analog signal into a digital signal. For example, an analog-digital converter converts an analog voltage into a digital code corresponding to the voltage level of the analog voltage.
In order to consider an analog-digital converter, an on-die thermal sensor including the analog-digital converter is described below. The on-die thermal sensor is an apparatus for controlling a period of a refresh operation of a semiconductor memory device according to a temperature.
FIG. 1 is a block diagram of a conventional on-die thermal sensor. The conventional on-die thermal sensor includes a band gap unit 100 and an analog-digital converter 110.
The band gap unit 100 senses a temperature by using a characteristic of a bipolar junction transistor (BJT). A base-emitter voltage Vbe of the BJT changes in the proportion of −1.8 mV/° C. per temperature changes. Accordingly, the band gap unit 100 amplifies the small change base-emitter voltage Vbe and outputs a first voltage Vtemp changing in the proportion of one to one per unit temperature change. As temperature increases, the first voltage Vtemp decreases.
The analog-digital converter 110 includes a voltage comparing unit 120, a counting unit 130, and a converting unit 140. The analog-digital converter 110 converts the first voltage Vtemp output from the band gap unit 100 into a thermal code which is a digital code.
The converting unit 140 is a digital-analog converter. The converting unit 140 outputs an analog second voltage DACOUT in response to the thermal code output from the counting unit 130. Voltages VULIMIT and VLLIMIT, input into the converting unit 140, determine maximum and minimum values of the second voltage DACOUT, respectively.
The voltage comparing unit 120 compares the first voltage Vtemp and the second voltage DACOUT. When the first voltage Vtemp is less than the second voltage DACOUT, the voltage comparing unit 120 outputs a decease signal DEC for decreasing the thermal code determined in the counting unit 130. When the first voltage Vtemp is higher than the second voltage DACOUT, the voltage comparing unit 120 outputs an increase signal INC for increasing the thermal code predetermined in the counting unit 130.
The counting unit 130 receives the increase signal INC or the decrease signal DEC from the voltage comparing unit 120. The counting unit 130 increases or decreases a thermal code determined therein and outputs a thermal code representing temperature.
That is, the analog-digital converter 110 compares the first voltage Vtemp with the second voltage DACOUT, and increases or decreases the thermal code repeatedly. Accordingly, the second voltage DACOUT tracks the first voltage Vtemp. When the tracking is finished, the thermal code becomes digital values corresponding to the first voltage Vtemp.
As described above, the analog-digital converter uses a method in which the second voltage DACOUT tracks the first voltage Vtemp. This kind of analog-digital converter is called a tracking analog-digital converter.
FIG. 2 is a block diagram of a conventional integrating analog-digital converter. The conventional integrating analog-digital converter includes an operational amplifier 210, a resistor R, a capacitor C, a comparator 220, an AND gate 230, and a counter 240.
The operational amplifier 210 integrates a negative reference voltage −Vref to output a second voltage Vout which increases as time passes. By counting clocks of a clock signal CLK input until the second voltage Vout reaches a first voltage Vin, the first voltage Vin is converted into a digital value.
The operational amplifier 210 performs the integration operation. An equation regarding a current passing through an inverting terminal of the operational amplifier 210 is described as follows.{(0−Vref)/R}=C{d(0−Vout)/dt}
That is, the operational amplifier 210 outputs the second voltage Vout which increases as time passes as shown in the following equation.Vout=(Vref/RC)*t 
The comparator 220 compares the second voltage Vout output from the operational amplifier 210 with the first voltage Vin to be converted into a digital value. When the first voltage Vin is higher than the second voltage Vout, the comparator 220 outputs a logic high level voltage. When the second voltage Vout is higher than the first voltage Vin, the comparator 220 outputs a logic low level voltage. Accordingly, while outputting a logic high level voltage at the initial operation, the comparator 220 outputs a logic low level voltage when the second voltage Vout becomes higher than the first voltage Vin as time passes.
The AND gate 230 receives an output of the comparator 220 and the clock signal CLK. When the output of the comparator 220 has a logic high level, the clock signal CLK is output to the counter 240. When the output of the comparator 220 has a logic low level, the output of the AND gate 230 becomes a logic low level regardless of a logic state of the clock signal CLK.
The counter 240 counts the number of logic high levels of the output of the AND gate 230 to generate a digital code. That is, the counter 240 counts clocks of the clock signal CLK until the second voltage Vout becomes higher than the first voltage Vin and generates the digital code. As described above, the analog type of first voltage Vin is converted into the digital code.
The integrating analog-digital converter shown in FIG. 2 can perform the converting operation with fewer errors than a tracking analog-digital converter. However, a semiconductor memory device hardly uses the integrating analog-digital converter because it is difficult to generate the negative reference voltage −Vref.
In the conventional integrating analog-digital converter shown in FIG. 2, the reference voltage input to the inverting terminal should be negative to output the second voltage Vout being positive. However, an additional circuit for pumping the negative voltage is required to generate the negative reference voltage −Vref. The additional circuit may increase a chip size and current consumption.